Antibodies are drawing attention as pharmaceuticals as they have long half-life in plasma and few adverse effects. Of them, a number of IgG-type antibody pharmaceuticals are available on the market and many antibody pharmaceuticals are currently under development (Non-patent Documents 1 and 2). Most of the antibody pharmaceuticals available on the market are chimeric antibodies, humanized antibodies, or human antibodies. Currently, many antibody pharmaceuticals are being developed which have more superior characteristics with improved drug efficacy, convenience, and cost from modification of humanized antibodies or human antibodies. Various technologies applicable to these antibody pharmaceuticals have been developed, including those that enhance effector function, antigen-binding activity, pharmacokinetics, or stability, and those that reduce the risk of immunogenicity. For methods of enhancing drug efficacy or reducing dosage, techniques that enhance antibody-dependent cell-mediated cytotoxicity (ADCC) or complement-dependent cytotoxicity (CDC) through amino acid substitution in the Fc domain of an IgG antibody have been reported (Non-patent Documents 3 and 4). Furthermore, affinity maturation has been reported as a technique for enhancing antigen-binding activity or antigen-neutralizing activity (Non-patent Document 5). This technique enables enhancement of antigen-binding activity through introduction of amino acid mutations into the CDR region of a variable region or such.
A problem encountered with current antibody pharmaceuticals is high production cost associated with the administration of extremely large quantities of protein. The preferred form of administration is thought to be subcutaneous formulation for chronic autoimmune diseases. In general, it is necessary that subcutaneous formulations are high concentration formulations. From the perspective of stability or such, the concentration limit for IgG-type antibody formulations is in general thought to be about 100 mg/ml (Non-patent Document 6). Low-cost, convenient antibody pharmaceuticals with more superior characteristics that can be administered subcutaneously in longer intervals can be provided by increasing the half-life of an antibody in the plasma to prolong its therapeutic effect and thereby reduce the amount of protein administered.
FcRn is closely involved in the long plasma half-life of antibodies. The plasma half-life of antibody is known to be different between antibody isotypes. IgG1 and IgG2 have the longest half-life in plasma, while IgG3 and IgG4 have a shorter half-life (Non-patent Document 7). A reported method for prolonging the plasma half-lives of IgG1 and IgG2 antibodies, which have a superior half-life in plasma, comprises substituting amino acids in the constant region to enhance the binding to FcRn (Non-patent Documents 8 to 10). However, introduction of artificial amino acid mutations into a constant region is encountered with the problem of immunogenicity. On the other hand, a method that comprises introducing amino acid mutations into antibody variable regions for improving antibody pharmacokinetics has recently been reported (Patent Document 1).
Patent Document 1 describes that the pharmacokinetics of IgG can be controlled by modifying its isoelectric point, and the plasma half-life of an antibody can be prolonged by reducing the isoelectric point of the antibody without loss of antigen-binding activity through introduction of amino acid substitutions into the antibody variable region framework. Specifically, the isoelectric point of an antibody can be reduced without loss of antigen-binding activity, for example, by introducing amino acid substitutions at H10, H12, H23, H39, H43, and H105, Kabat's numbering. It is also possible to introduce amino acid mutations into other framework sequences without loss of binding activity. In some cases, however, introduction of amino acid substitutions into framework sequences alone is thought to be insufficient for significant reduction of isoelectric point. This is because a human antibody sequence is generally used as a framework sequence after amino acid substitution to reduce immunogenicity, but human antibody framework sequences are highly conserved and have low diversity, and thus there is little flexibility for amino acid substitution. Therefore, when amino acid substitution into the framework alone is insufficient to reduce the isoelectric point of an antibody, it would be difficult to further reduce the isoelectric point.
In contrast, CDR sequences have an enormous diversity due to somatic mutations, and because they have the diversity needed for acquiring antigen-binding activity, there is significantly greater flexibility for amino acid substitution compared to framework sequences. However, amino acid substitution in CDR sequence is generally known to affect the antigen-binding activity of an antibody, as CDR sequence is the most important factor for strong antigen-binding activity. Thus, it is difficult to reduce the isoelectric point of an antibody by substituting amino acids in its CDR sequence without considerable loss of antigen-binding activity. Furthermore, CDR sequence varies greatly depending on the type of antigen; thus, regardless of antibody specificity, it has been believed to be very difficult to substitute amino acids in an antibody CDR sequence without considerable loss of the antibody's antigen-binding activity. In fact, this can be inferred from many findings described below.
In general, antibodies derived from a nonhuman animal species are humanized by CDR grafting, in which a human framework sequence is grafted to the CDR sequence of the nonhuman animal species. If a humanized antibody obtained by CDR grafting does not have a comparable binding activity as the chimeric antibody, the binding activity can be recovered from substituting a portion of the framework sequence which determines the CDR structure with amino acids of the antibody framework sequence of the nonhuman animal species from which the antibody is derived (Non-patent Document 11). The CDR sequence and structure are very important for the antigen-binding activity and specificity of an antibody. Furthermore, the antigen-binding activity of an antibody is generally known to be reduced when antibody CDR residues are modified by isomerization of aspartic acid residues, deamidation of aspartic acid residues, or oxidation of methionine residues in antibody CDR (Non-patent Document 12), and this also suggests that CDR sequence is very important for the antigen-binding activity of antibodies. In addition, it has been further reported that not only the antigen-binding activity but also the expression level of an antibody is often considerably reduced when amino acid substitutions are introduced into the heavy chain CDR2 sequence of an antibody (Non-patent Documents 13 to 15). In particular, the expression level of an antibody is known to be markedly reduced when amino acid substitution is introduced at H51 (Non-patent Document 16). Furthermore, the antigen-binding activity has been reported to be considerably reduced in almost all cases when mutations are introduced into the heavy chain CDR3 sequence of an antibody (Non-patent Documents 17 and 18). Alternatively, the antigen-binding activity of an antibody is often markedly reduced when amino acids in the antibody CDR are substituted with alanine by alanine scanning mutagenesis (Non-patent Documents 19 to 23). The effect of alanine substitution on antigen-binding activity is thought to depend on antibody specificity. In sum, the antigen-binding activity of an antibody is generally considered to be reduced by amino acid substitution in the CDR sequence, and there is no previous report on positions of amino acids whose substitution does not significantly reduce the antigen-binding activity of an antibody regardless of its antibody specificity.
In antibody engineering to produce antibody molecules with more superior characteristics, almost all amino acid substitutions introduced into antibody CDR sequences are aimed at affinity maturation. Affinity maturation is a method for obtaining antibodies with improved antigen-binding activity, and is generally conducted by displaying on phages or ribosomes an antibody library comprising randomized CDR sequences derived from the CDR sequences of a parent antibody molecule and panning on the antigen. This method enables discovery of amino acid substitutions in antibody CDR sequence that improve antigen-binding activity (Non-patent Documents 5 and 24 to 26). However, amino acid substitutions found by the above-described method which improve antigen-binding activity are different depending on the antibody specificity. Thus, there is no previous report on positions of amino acids in CDR sequence whose substitution improves the antigen-binding activity regardless of the antibody specificity. Other than affinity maturation for modifying CDR sequence, methods for improving expression levels of antibodies in mammalian cells by substituting amino acids at specific positions in the CDR sequence (Patent Document 2) are reported. According to Patent Document 2, the expression levels of antibodies in mammalian cells can be improved independently of the antibody specificity by substituting amino acids at specific positions in the CDR sequence with a particular sequence. Alternatively, some reports describe deimmunization where the immunogenicity of an antibody is reduced by avoiding T cell epitopes in the antibody CDR sequence. However, there is no previous report on methods for substituting amino acids in an antibody, regardless of its antibody specificity, to remove T cell epitopes from the CDR sequence without loss of binding activity (Non-patent Documents 27 and 28).
As described above, the antibody CDR sequence is closely involved in antigen binding. Therefore, amino acid substitutions in CDR sequence generally impair binding activity. The effect of amino acid substitution in CDR sequence on antigen binding differs depending on the antibody specificity. Patent Document 1 describes some examples on the control of isoelectric point by amino acid substitution in CDR; however, the antigen-binding activity can be impaired in some kinds of antibodies. Alternatively, methods have been reported for improving the expression of antibodies independently of the antibody specificity by common amino acid substitutions; however, there is no previous report on methods for improving an antibody's antigen-binding activity or removing T cell epitopes without considerable loss of an antibody's antigen-binding activity. There is absolutely no report on antibody CDR sequences whose amino acids can be substituted without considerable loss of the antibody's antigen-binding activity regardless of the antibody specificity.
Documents of related prior arts for the present invention are described below.    [Non-patent Document 1] Janice M Reichert, Clark J Rosensweig, Laura B Faden & Matthew C Dewitz. Monoclonal antibody successes in the clinic. Nature Biotechnology (2005) 23:1073-1078    [Non-patent Document 2] Pavlou A K, Belsey M J. The therapeutic antibodies market to 2008. Eur J Pharm Biopharm. 2005 April; 59(3):389-96    [Non-patent Document 3] Presta L G. Engineering of therapeutic antibodies to minimize immunogenicity and optimize function. Adv Drug Deliv Rev. 2006 Aug. 7; 58(5-6):640-56    [Non-patent Document 4] Kim S J, Park Y, Hong H J. Antibody engineering for the development of therapeutic antibodies. Mol Cells. 2005 Aug. 31; 20(1):17-29 Review    [Non-patent Document 5] Fujii I. Antibody affinity maturation by random mutagenesis. Methods Mol Biol. (2004) 248:345-59    [Non-patent Document 6] Shire S J, Shahrokh Z, Liu J. Challenges in the development of high protein concentration formulations. J Pharm Sci. 2004 June; 93(6):1390-402    [Non-patent Document 7] Salfeld J G. Isotype selection in antibody engineering. Nat Biotechnol. 2007 December; 25(12):1369-72    [Non-patent Document 8] Hinton P R, Johlfs M G, Xiong J M, Hanestad K, Ong K C, Bullock C, Keller S, Tang M T, Tso J Y, Vasquez M, Tsurushita N. Engineered human IgG antibodies with longer serum half-lives in primates. J Biol Chem. 2004 Feb. 20; 279(8):6213-6    [Non-patent Document 9] Hinton P R, Xiong J M, Johlfs M G, Tang M T, Keller S, Tsurushita N. An engineered human IgG1 antibody with longer serum half-life. J Immunol. 2006 Jan. 1; 176(1):346-56    [Non-patent Document 10] Ghetie V, Popov S, Borvak J, Radu C, Matesoi D, Medesan C, Ober R J, Ward E S. Increasing the serum persistence of an IgG fragment by random mutagenesis. Nat Biotechnol. 1997 July; 15(7):637-40    [Non-patent Document 11] Almagro J C, Fransson J. 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J Biol Chem. 1997 Oct. 24; 272(43):26864-70    [Non-patent Document 19] Gerstner R B, Carter P, Lowman H B. Sequence plasticity in the antigen-binding site of a therapeutic anti-HER2 antibody. J Mol Biol. 2002 Aug. 30; 321(5):851-62    [Non-patent Document 20] Vajdos F F, Adams C W, Breece T N, Presta L G, de Vos A M, Sidhu S S. Comprehensive functional maps of the antigen-binding site of an anti-ErbB2 antibody obtained with shotgun scanning mutagenesis. J Mol Biol. 2002 Jul. 5; 320(2):415-28    [Non-patent Document 21] Pons J, Rajpal A, Kirsch J F. Energetic analysis of an antigen/antibody interface: alanine scanning mutagenesis and double mutant cycles on the HyHEL-10/lysozyme interaction. 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Phage display: a molecular tool for the generation of antibodies—a review. Placenta. 2000 March-April; 21 Suppl A:S106-12    [Non-patent Document 26] Rajpal A, Beyaz N, Haber L, Cappuccilli G, Yee H, Bhatt R R, Takeuchi T, Lerner R A, Crea R. A general method for greatly improving the affinity of antibodies by using combinatorial libraries. Proc Natl Acad Sci USA. 2005 Jun. 14; 102(24):8466-71    [Non-patent Document 27] De Groot A S, Knopp P M, Martin W. De-immunization of therapeutic proteins by T-cell epitope modification. Dev Biol (Basel). (2005) 122:171-94    [Non-patent Document 28] www.algonomics.com/proteinengineering/tripole_applications.php    [Patent Document 1] WO/2007/114319    [Patent Document 2] US/2006/0019342